The million dollar question??? Yes, there are thousands of old books out there. I spent a great deal of money and time searching and will continue the rest of my life looking at books on longevity secrets.
How can you tell a good one? If you haven't read a wide variety and experimented with self-applied health, you won't have much luck. This is why I started my little School of Self-Applied Prevention. I start out with the book that changed my life and that is just about all people need to be free. I then have much better books that explain longevity secrets and advanced methods of all kinds. I have tried just about everything I have read and only promote what works, this is something that takes time and experience.
The idea is to spark the interest of people into the art of living drug free and healthy for many many years! My ideas should not be your methods, you should read and make your own experiments and do what works for you. I make herbal solutions based on what I read and do many things I don't expect others will ever try.
You can start with my schooling or search on your own and have alot of fun doing it. I figure I wasted at least $25,000 and 15 years before I found my first real book that had the cure that used no drugs or herbs and works for everyone all the time, then it seemed like these types of books just started coming my way and I have quite a nice collection and they all have one thing in commom, they cure at no cost. I studied herbal books for 10 years and did many great things, but never learned the cure until I came accross the right medical books.
Good Luck on your search, I am sure there are hundreds out there I will never read in my lifetime.